Method and apparatus for handling circuit switched calls at a user equipment

ABSTRACT

A method at a user equipment including detecting that a circuit switched (CS) call has been requested at the user equipment; if a most recent registration attempt or registration update attempt with a network of a first radio access technology (RAT) resulted in an abnormal state, registering with a network of a second RAT; and initiating the CS call with the network of the second RAT.

FIELD OF THE DISCLOSURE

The present disclosure is directed in general to circuit switched calls,and more specifically, to mobile originated circuit switched callperformance.

BACKGROUND

The Third Generation Partnership Project (3GPP) defines a long-termevolution (LTE) architecture, which provides high data rate, lowlatency, packet optimization and improved system capacity and coverage.In an LTE system, an evolved universal terrestrial radio access network(E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) andcommunicates with a plurality of mobile stations, also referred to asuser equipments (UEs).

LTE technology supports packet based services only. However fallback isspecified for circuit switched (CS) services as well. CS Fallback (CSFB)defines a mechanism for using a CS legacy network (e.g. the GlobalSystem for Mobile Communications (GSM)/Wideband Code Division MultipleAccess (WCDMA)) to provide voice and traditional CS-domain services(e.g. voice call, a CS short message service (SMS), UnstructuredSupplementary Service Data (USSD), among others). To provide these CSservices, a UE may reuse the CS infrastructure when the UE is served byE-UTRAN.

When a CSFB capable UE selects an E-UTRAN, there is a combined TrackingArea Update/Location Area Update (TAU/LAU) with an international mobilesubscriber identity (IMSI) attach procedure initiated. Further, when aCSFB capable UE camping on an E-UTRAN selects a tracking area that isdifferent from one stored in it, there is a combined TA/LA update.Further, when a timer for periodic TA/LA expires, there is a combinedTA/LA updating. Such procedures may result in mobile originated (MO)call failure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present systems and methods will be better understood with referenceto the drawings, in which:

FIG. 1 is a block diagram of a mobile communication system;

FIG. 2 is a data flow diagram showing an example circuit switchedfallback process;

FIG. 3 a flow diagram showing one method according to the presentdisclosure where a CS mobile originated call fails;

FIG. 4 is a flow diagram showing an embodiment of the present disclosurewhere the UE re-registers to prevent CS call failure; and

FIG. 5 is a block diagram of an example UE capable of implementing themethods and systems of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure provides a method at a user equipment comprising:detecting that a circuit switched (CS) call has been requested at theuser equipment; if a most recent registration attempt or registrationupdate attempt with a network of a first radio access technology (RAT)resulted in an abnormal state, registering with a network of a secondRAT; and initiating the CS call with the network of the second RAT.

The present disclosure further provides a user equipment comprising aprocessor configured to: detect that a circuit switched (CS) call hasbeen requested at the user equipment; if a most recent registrationattempt or registration update attempt with a network of a first radioaccess technology (RAT) resulted in an abnormal state, register with anetwork of a second RAT; and initiate the CS call with the network ofthe second RAT.

The long term evolution architecture is primarily packet based and thismay present a challenge for circuit-switched services such as voice orshort message service (SMS). One option to deal with such limitation inLTE is referred to as circuit switched fallback (CSFB), where a userequipment will fallback to a network using a second generation (2G) or athird generation (3G) radio access technology (RAT) for circuit switchedcalls while camping on the LTE network for packet switched services.

The UE will fallback to the 2G or 3G RAT on either a mobile originated(MO) or a mobile terminated (MT) call.

In order to accommodate the circuit switched fallback, the mobileswitching center (MSC) server for the 2G or 3G RAT communicates with themobility management entity (MME) for the LTE network.

Circuit switched fallback is provided for in the 3GPP LTE TechnicalSpecification (TS) 23.272, “Third Generation Partnership Project;Technical Specification Group Services and System Aspects; CircuitSwitched (CS) Fallback in Evolved Packet System (EPS); Stage 2”; v.11.5.0, June 2013, the contents of which are incorporated herein byreference. In particular the system architecture is provided in FIGS.4.2-1 of the 3GPP TS 23.272 specification and is reproduced herein withregard to FIG. 1.

Reference is now made to FIG. 1, which illustrates one embodiment of amobile communication system that includes a 2G/3G RAT, as well as an LTERAT.

In particular, a UE 110 is a multi-mode UE capable of communicating withboth a network of an LTE RAT as well as a network of a 2G/3G RAT. Asseen in FIG. 1, UE 110 communicates with either, or both of a universalmobile telecommunications system (UMTS) terrestrial radio access network(UTRAN) 120 or a GSM Edge Radio Access Network (GERAN) 122 for circuitswitched services. UE 110 further communicates with an evolved-UTRAN(E-UTRAN) 130 for the LTE services.

Both UTRAN 120 and GERAN 122 communicate with a serving general packetradio service (GPRS) support node (SGSN) 140 for packet switchedservices and with MSC server 160 for circuit switched services.

The E-UTRAN communicates with MME 150.

In the CSFB system, MSC server 160 further communicates with MME 150.

During normal operation, UE 110 will camp on the LTE system. If a mobileterminated call is received then MSC server 160 will notify MME 150,which will then send a page to UE 110 through E-UTRAN 130 to transitionto UTRAN 120 or GERAN 122. The UE 110 will then transition for theduration of the circuit switched call. At the end of the circuit switchcall the UE then transitions back to the LTE system.

Packet switched services may either be suspended for the duration of thecircuit switched call or may be transferred to SGSN 140, where the UE110 will continue to receive packet data, although at a lower speed thanfrom the LTE system.

For a mobile originated call, the UE 110 may transition to UTRAN 120 orGERAN 122 for the duration of the call. Again, packet switched servicesmay either be suspended or transitioned to SGSN 140.

The typical process for a mobile originated call (without PS handoverfor simplicity) is provided in FIG. 2. FIG. 2 follows FIGS. 6.3-1 of the3GPP TS 23.272 Specification.

In particular, a UE 210 communicates with a LTE eNB 212, which in turncommunicates with MME 214.

The fallback for UE 210 has a base station subsystem/radio networksubsystem (BSS/RNS) 216 which is controlled by MSC 220. Data on thefallback RAT is controlled through SGSN 222 and bearers are controlledusing serving gateway/packet data network gateway (SGW/PGW) 224.

For a fallback based on a mobile originated call, UE 210 first providesa non-access spectrum (NAS) extended service request 230 to eNB 212,which forwards the message to MME 214.

In response, the MME provides a UE context modification request with aCS fallback indicator, as shown by message 232. The eNB 212 sends aresponse message 234 back to MME 214.

Depending on the configuration, the MME 214 may also send a message 236to MSC 220 indicating that a CM service request message is due.

As shown by block 240, an optional measurement report may be solicitedby MME 214 to determine a target GERAN/UTRAN cell for redirection.

In the embodiment of FIG. 2, it is assumed that a network supportsinter-RAT cell change and thus eNB 212 may send message 242 to providethe inter-RAT cell change to a neighbor GERAN cell, for example. Themessage may optionally contain a network assisted cell change (NACC).

eNB 212 may then send a UE context release request to MME 214, as shownby message 244.

The UE's context is then released, as shown by block 246. The UE thenchanges RATs and may perform one of, or both of, a location area update(LAU) and a routing area update (RAU), as shown by block 250.

The UE then suspends packet services by sending a suspend message 252 toBSS/RNS 216 which forwards the message to SGSN 222. The suspension isthen coordinated between MME 214 and SGSN 222, as shown by arrow 254 andthe bearers are then updated, as shown by arrow 256. As indicated above,the example of FIG. 2 is for no PS handover.

At this point, UE 210 sends a CM service request to BSS/RNS 216, asshown by arrow 260 and BSS/RNS 216 sends the circuit switched messagewith the CM service request to MSC 220, as shown by arrow 262.

If the service request is rejected with reject cause IMSI unknown invisitor location registry (VLR), the process proceeds into block 270.For example, this may occur if the MSC has changed and the UE did noupdate its registration. In other cases, after the rejection with thecause IMSI unknown in VLR, any mobile terminated call subsequent to thismay also fail due to the unknown registration state of the device.

If the process proceeds into block 270 then a service reject message 272is forwarded from MSC 220, through BSS/RNS 216 to UE 210 and a locationarea update, as shown by block 274 is then performed.

After the location area update, or if no service rejection is received,the mobile originated call may occur as shown by arrow 280.

After the call, a routing area update may occur, as shown by block 290.

As will be appreciated, the rejection of the service at block 270 willcause the MO call to fail, and may cause poor user experience. One causeof a rejection may be the UE and network elements being in differentstates, as provided below.

Reference is now made to FIG. 3, where the process begins at block 310.In block 310 the UE starts with a successful LAU procedure in a 2G/3GRAT such as a UTRAN/GERAN establishing a public land mobilenetwork/location area code (PLMN/LAC) or the UE being successfullycamped in E-UTRAN.

The process next proceeds to block 312 in which a check is made todetermine one or more of three factors: 1) whether the UE is configuredto transition from a state of having a RRC connection with theUTRAN/GERAN system to a state of having RRC connection with the LTEsystem. In other words, the UE has an Inter-RAT re-selection to E-UTRAN;2) whether the UE selects a non-stored TA; and 3) whether the timer forperiodic TA/LA updating has expired. If no, the process loops at block312, and otherwise to process proceeds to block 314.

As a result of Inter-RAT re-selection at block 312, a combined TAU/LAUwith IMSI attach procedure may be initiated to register the UE. In somecases, the TAU/LAU may fail due to radio link failure RLF, release ofRRC (radio resource control protocol) connection from the RAT, or anyother reason. The result may be abnormal termination of theregistration. In other cases, combined tracking area (TA)/location area(LA) updating may result in the abnormal termination.

In the example of FIG. 3, it is assumed that abnormal termination of theregistration occurred, as shown by block 314. The UE will detect theabnormal termination of the registration and may, according to oneembodiment of the present disclosure, consider the UE to be in anabnormal state with regards to registration. As used herein, the“abnormal state” refers to any state in which the UE and the network arenot properly registered with each other, and may, for example, be due toRLF, release of RRC connection, or other causes during the registrationprocess.

As a result of the abnormal termination of registration at block 314,the Evolved Packet System (ESP) mobility management (EMM) state is setto REGISTERED and the EMM sub state is EMM_REGISTERED_ATTEMPTINGTO_UPDATE.

The CSFB call bit may be set to TRUE because the active RAT is E-UTRAN.Therefore, when a CS-domain service is requested the UE will undergoCSFB to establish CS-domain service.

A check at block 318 is then made for a MO CS-call (or other CS-domainservice) request. If no MO CS-call is made, the process proceeds back toblock 314.

If a MO CS call request is made, since the registration was abnormallyterminated, a PLMN selection may be initiated in the gateway due to theMO call request. However, in this case the PLMN/LAC found may be thesame as the UE had at the last successful registration at block 310. Asa result no location update is initiated.

Based on the above, the UE and network registration may beunsynchronized as a result of abnormal termination of the registrationat block 314. The network may have deregistered the UE IMSI due to theprevious TAU attempt that did not complete. The UE, on the other hand,assumes it has successfully registered in the same PLMN/LAC as in block310.

The TAU procedure was unsuccessful during the Inter-RAT reselection toE-UTRAN, therefore, the EMM was not fully registered to replace theregistration information from the UTRAN/GERAN. The EMM may be in ATTEMPTTO UPDATE state and cannot handle the MO call request at block 318.

As a result of the MO call request the UE may initiate the callestablishment procedure (CM SERVICE REQUEST 260 from FIG. 2) at block320. The network may reject the CS call attempt (CM SERVICE REJECT,cause 4, IMSI unknown in VLR), as received by the UE at block 322.

As a result of the call request at block 320 and call failure at block322 the UE may initiate a LAU request and re-register successfully, asshown by block 324. However, in this case the call attempt has alreadybeen aborted or has failed, resulting in a poor user experience.

Based on the above, although the radio resource release during thecombined TAU/LAU IMSI attach (or combined TA/LA updating) is triggeredby the network conditions, according to one embodiment of the presentdisclosure the UE may handle the conditions in order to establish thevoice call successfully. In particular, in accordance with the presentdisclosure, the non-access stratum state machine at the UE may bemodified to ensure that if a registration attempt is abnormally endedwhile a voice call attempt is detected, the UE may initiate a furtherregistration attempt, even if the tracking area code (TAC)/location areacode (LAC) provided in a broadcast channel message is the same as thelast successfully registered TAC/LAC.

Reference is now made to FIG. 4. In particular, the process of FIG. 4starts at block 410 with a precondition that the UE starts with asuccessful LAU procedure in a 2G/3G RAT such as a UTRAN/GERANestablishing a public land mobile network/location area code (PLMN/LAC)or the UE is successfully camped in E-UTRAN. The process then proceedsto block 412 in which a check is made to determine one or more of threefactors: 1) whether the UE is configured to transition from a state ofhaving a RRC connection with the UTRAN/GERAN system to a state of havingRRC connection with the LTE system. In other words, the UE has anInter-RAT re-selection to E-UTRAN; 2) whether the UE selects anon-stored TA; and 3) whether the timer for periodic TA/LA updating hasexpired. If no, the process loops at block 412, and otherwise to processproceeds to block 414.

As a result of Inter-RAT re-selection at block 412, a combined TAU/LAUwith IMSI attach procedure may be initiated to register the UE. In somecases, the TAU/LAU may fail due to radio link failure RLF, release ofRRC (radio resource control protocol) connection from the network, orany other reason. The result may be abnormal termination of theregistration. In other cases, combined tracking area (TA)/location area(LA) updating may result in the abnormal termination.

Again, in the example of FIG. 4, it is assumed that abnormal terminationof the registration occurred, as shown by block 414. The UE will detectthe abnormal termination of the registration and may, according to oneembodiment of the present disclosure, consider the UE to be in anabnormal state with regards to registration.

As a result of the abnormal termination of registration at block 414,the Evolved Packet System (ESP) mobility management (EMM) state is setto REGISTERED and the EMM sub state is EMM_REGISTERED_ATTEMPTINGTO_UPDATE.

The CSFB call bit may be set to TRUE because the active RAT is E-UTRAN.Therefore, when a CS-domain service is requested the UE will undergoCSFB to establish CS-domain service.

A check at block 418 is then made for a MO CS-call (or other CS-domainservice) request. If no MO CS-call is made, the process proceeds back toblock 314.

If a MO CS call request is made the process proceeds from block 418 toblock 420 in which a registration attempt with a network of the 2G/3GRAT is initiated. The registration attempt may be in the form of an LAU.Further, a follow_on_request flag may optionally be used to instruct anetwork not to release the radio resource when the LAU proceed iscomplete. In fact in the LAU accepted message, the network will set acorresponding flag (follow_on_proceed) to 1 to let the UE know that itcan now send the CM_SERVICE_REQUEST. If the follow_on_request flag isnot set to 1, the network will release the radio resource connectionafter the LAU accepted. Thus the follow_on_request flag may be used tooptimize the registration process when a CS call is pending.

In one embodiment a check may be made after the registration at block420 to ensure success and if the registration is not successful theprocess could be ended or block 420 could be repeated.

From block 420 the process proceeds to block 422 in which the pendingcall is initiated. In this case, since registration occurs, the CS callestablishment succeeds in the embodiment of FIG. 4, avowing the pooruser experience described above with regards to FIG. 3.

The above embodiments may be implemented by any UE. One exemplary deviceis described below with regard to FIG. 5.

UE 500 is typically a two-way wireless communication device having voiceand/or data communication capabilities. UE 500 generally has thecapability to communicate with other computer systems on the Internet.Depending on the exact functionality provided, the UE may be referred toas a data messaging device, a two-way pager, a wireless e-mail device, acellular telephone with data messaging capabilities, a wireless Internetappliance, a wireless device, a mobile device, or a data communicationdevice, as examples.

Where UE 500 is enabled for two-way communication, it may incorporate acommunication subsystem 511, including both a receiver 512 and atransmitter 514, as well as associated components such as one or moreantenna elements 516 and 518, local oscillators (LOs) 513, and aprocessing module such as a digital signal processor (DSP) 520. As willbe apparent to those skilled in the field of communications, theparticular design of the communication subsystem 511 will be dependentupon the communication network in which the device is intended tooperate. The radio frequency front end of communication subsystem 511can be any of the embodiments described above.

Network access requirements will also vary depending upon the type ofnetwork 519. In some networks network access is associated with asubscriber or user of UE 500. A UE may require a removable user identitymodule (RUIM) or a subscriber identity module (SIM) card in order tooperate on a network. The SIM/RUIM interface 544 is normally similar toa card-slot into which a SIM/RUIM card can be inserted and ejected. TheSIM/RUIM card can have memory and hold many key configurations 551, andother information 553 such as identification, and subscriber relatedinformation.

When required network registration or activation procedures have beencompleted, UE 500 may send and receive communication signals over thenetwork 519. As illustrated in FIG. 5, network 519 can consist ofmultiple base stations communicating with the UE. Further, as describedabove, a multi-mode UE may communicate with base stations of differentnetworks 519, for example a WCDMA and an LTE network.

Signals received by antenna 516 through communication network 519 areinput to receiver 512, which may perform such common receiver functionsas signal amplification, frequency down conversion, filtering, channelselection and the like. A/D conversion of a received signal allows morecomplex communication functions such as demodulation and decoding to beperformed in the DSP 520. In a similar manner, signals to be transmittedare processed, including modulation and encoding for example, by DSP 520and input to transmitter 514 for digital to analog conversion, frequencyup conversion, filtering, amplification and transmission over thecommunication network 519 via antenna 518. DSP 520 not only processescommunication signals, but also provides for receiver and transmittercontrol. For example, the gains applied to communication signals inreceiver 512 and transmitter 514 may be adaptively controlled throughautomatic gain control algorithms implemented in DSP 520.

UE 500 generally includes a processor 538 which controls the overalloperation of the device. Communication functions, including data andvoice communications, are performed through communication subsystem 511.Processor 538 also interacts with further device subsystems such as thedisplay 522, flash memory 524, random access memory (RAM) 526, auxiliaryinput/output (I/O) subsystems 528, serial port 530, one or morekeyboards or keypads 532, speaker 534, microphone 536, othercommunication subsystem 540 such as a short-range communicationssubsystem and any other device subsystems generally designated as 542.Serial port 530 could include a USB port or other port known to those inthe art.

Some of the subsystems shown in FIG. 5 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 532 and display522, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the processor 538 may be stored in apersistent store such as flash memory 524, which may instead be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile memory such as RAM 526. Received communication signals may alsobe stored in RAM 526.

As shown, flash memory 524 can be segregated into different areas forboth computer programs 558 and program data storage 550, 552, 554 and556. These different storage types indicate that each program canallocate a portion of flash memory 524 for their own data storagerequirements. Processor 538, in addition to its operating systemfunctions, may enable execution of software applications on the UE. Apredetermined set of applications that control basic operations,including at least data and voice communication applications forexample, will normally be installed on UE 500 during manufacturing.Other applications could be installed subsequently or dynamically.

Applications and software may be stored on any computer readable storagemedium. The computer readable storage medium may be a tangible or intransitory/non-transitory medium such as optical (e.g., CD, DVD, etc.),magnetic (e.g., tape) or other memory known in the art.

One software application may be a personal information manager (PIM)application having the ability to organize and manage data itemsrelating to the user of the UE such as, but not limited to, e-mail,calendar events, voice mails, appointments, and task items. Naturally,one or more memory stores would be available on the UE to facilitatestorage of PIM data items. Such PIM application may have the ability tosend and receive data items, via the wireless network 519. Furtherapplications may also be loaded onto the UE 500 through the network 519,an auxiliary I/O subsystem 528, serial port 530, short-rangecommunications subsystem 540 or any other suitable subsystem 542, andinstalled by a user in the RAM 526 or a non-volatile store (not shown)for execution by the processor 538. Such flexibility in applicationinstallation increases the functionality of the device and may provideenhanced on-device functions, communication-related functions, or both.For example, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing the UE 500.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem511 and input to the processor 538, which may further process thereceived signal for output to the display 522, or alternatively to anauxiliary I/O device 528.

A user of UE 1500 may also compose data items such as email messages forexample, using the keyboard 532, which may be a complete alphanumerickeyboard or telephone-type keypad, or a virtual keyboard, among others,in conjunction with the display 522 and possibly an auxiliary I/O device528. Such composed items may then be transmitted over a communicationnetwork through the communication subsystem 511.

For voice communications, overall operation of UE 500 is similar, exceptthat received signals would typically be output to a speaker 534 andsignals for transmission would be generated by a microphone 536.Alternative voice or audio I/O subsystems, such as a voice messagerecording subsystem, may also be implemented on UE 500. Although voiceor audio signal output is generally accomplished primarily through thespeaker 534, display 522 may also be used to provide an indication ofthe identity of a calling party, the duration of a voice call, or othervoice call related information for example.

Serial port 530 in FIG. 5 may be implemented in a personal digitalassistant (PDA)-type UE for which synchronization with a user's desktopcomputer (not shown) may be desirable, but is an optional devicecomponent. Such a port 530 would enable a user to set preferencesthrough an external device or software application and would extend thecapabilities of UE 500 by providing for information or softwaredownloads to UE 500 other than through a wireless communication network.The alternate download path may for example be used to load anencryption key onto the device through a direct and thus reliable andtrusted connection to thereby enable secure device communication. Aswill be appreciated by those skilled in the art, serial port 530 canfurther be used to connect the UE to a computer to act as a modem or forcharging purposes.

Other communications subsystems 540, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between UE 500 and different systems ordevices, which need not necessarily be similar devices. For example, thesubsystem 540 may include an infrared device and associated circuits andcomponents or a Bluetooth™ communication module to provide forcommunication with similarly enabled systems and devices. Subsystem 540may further include non-cellular communications such as WiFi or WiMAX,or near field communications (NFC).

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

Various exemplary clauses may include:

AA. A method at a user equipment comprising: detecting that a circuitswitched (CS) call has been requested at the user equipment; if a mostrecent registration attempt or registration update attempt with anetwork of a first radio access technology (RAT) resulted in an abnormalstate, registering with a network of a second RAT; and initiating the CScall with the network of the second RAT.

BB. The method of clause AA, wherein the first RAT is a packet switchedonly RAT.

CC. The method of clause AA or clause BB, further comprising reselectingto the network of the second RAT for the CS call.

DD. The method of clause CC, wherein the reselecting to the network ofthe second RAT includes registering with the network of the second RAT.

EE. The method of clause CC, wherein the registering with the network ofthe second RAT is performed even if a tracking area code or a locationarea code of a last successful network registration matches the trackingarea code or location area code in a broadcast message from the networkof the second RAT.

FF. The method of clause EE, wherein the registering is in the form of alocation area update that utilizes a follow_on_request flag.

GG. The method of clause CC, wherein the reselecting comprises sending aCM service request to the network of the second RAT.

HH. The method of any one of clauses AA to GG, wherein the most recentregistration attempt with the network of the first RAT is in an abnormalstate due to a radio link failure or a radio resource control connectionrelease.

II. The method of clause HH, wherein the abnormal state is based on areject cause indicating an identifier is unknown in a visitor locationregistry.

JJ. The method of any one of clauses AA to II, further comprising, afterthe registering with the network of the second RAT, checking whether there-registration is successful, and if not ending the process.

KK. The method of any one of clauses AA to JJ, further comprising, afterthe registering with the network of the second RAT, checking whether there-registration is successful, and if not proceeding back to registeringwith the network of the second RAT.

LL. The method of any one of clauses AA to KK, wherein the first RAT isan evolved-universal terrestrial radio access (E-UTRAN) RAT.

MM. The method of any one of clauses AA to LL, wherein the second RAT isa global system for mobile communications (GSM) RAT or universalterrestrial radio access network (UTRAN) RAT.

NN. The method of any one of clauses AA to MM, wherein the circuitswitched call is a voice call.

OO. A user equipment comprising a processor configured to: detect that acircuit switched (CS) call has been requested at the user equipment; ifa most recent registration attempt or registration update attempt with anetwork of a first radio access technology (RAT) resulted in an abnormalstate, register with a network of a second RAT; and initiate the CS callwith the network of the second RAT.

PP. The user equipment of clause OO, wherein the first RAT is a packetswitched only RAT.

QQ. The user equipment of clause OO or PP, wherein the user equipment isconfigured to reselect to the network of the second RAT for the CS call.

RR. The user equipment of clause QQ, wherein the reselecting to thenetwork of the second RAT includes registering with the network of thesecond RAT.

SS. The user equipment of clause QQ, wherein the processor is configuredto register with the network of the second RAT even if a tracking areacode or a location area code of a last successful registration matchesthe tracking area code or location area code in a broadcast message fromthe network of the second RAT.

TT. The user equipment of clause QQ, wherein the registration with thenetwork of the second RAT is in the form of a location area update thatutilizes a follow_on_request flag.

UU. The user equipment of clause QQ, wherein the processor is configuredto reselect by sending a CM service request to the network of the secondRAT.

VV. The user equipment of any one of clauses OO to UU, wherein the mostrecent registration attempt with the network of the first RAT resultedin an abnormal state due to a radio link failure or a radio resourcecontrol connection release.

WW. The user equipment of clause VV, wherein the abnormal state is basedon a reject cause indicating an identifier is unknown in a visitorlocation registry.

XX. The user equipment of any one of clauses OO to WW, wherein theprocessor is further configured to, after the registering with thenetwork of the second RAT, check whether the registration is successful,and if not end the process.

YY. The user equipment of any one of clauses OO to XX, wherein theprocessor is further configured to, after the registering with thenetwork of the second RAT, check whether the registration is successful,and if not proceeding back to register with the network of the secondRAT.

ZZ. The user equipment of any one of clauses OO to YY, wherein the firstRAT is an evolved-universal terrestrial radio access (E-UTRAN) RAT.

AAA. The user equipment of any one of clauses OO to ZZ, wherein thesecond RAT is a global system for mobile communications (GSM) RAT oruniversal terrestrial radio access network (UTRAN) RAT.

BBB. The user equipment of any one of clauses OO to AAA, wherein thecircuit switched call is a voice call.

1. A method at a user equipment comprising: detecting that a circuitswitched (CS) call has been requested at the user equipment; if a mostrecent registration attempt or registration update attempt with anetwork of a first radio access technology (RAT) resulted in an abnormalstate, registering with a network of a second RAT; and initiating the CScall with the network of the second RAT.
 2. The method of claim 1,wherein the first RAT is a packet switched only RAT.
 3. The method ofclaim 1, further comprising reselecting to the network of the second RATfor the CS call.
 4. The method of claim 3, wherein the reselecting tothe network of the second RAT includes the registering with the networkof the second RAT.
 5. The method of claim 3, wherein the registeringwith the network of the second RAT is performed even if a tracking areacode or a location area code of a last successful network registrationmatches the tracking area code or location area code in a broadcastmessage from the network of the second RAT.
 6. The method of claim 5,wherein the registering is in the form of a location area update thatutilizes a follow_on_request flag.
 7. The method of claim 3, wherein thereselecting comprises sending a CM service request to the network of thesecond RAT.
 8. The method of claim 1, wherein the most recentregistration attempt with the network of the first RAT is in an abnormalstate due to a radio link failure or a radio resource control connectionrelease.
 9. The method of claim 8, wherein the abnormal state is basedon a reject cause indicating an identifier is unknown in a visitorlocation registry.
 10. The method of claim 1, further comprising, afterthe registering with the network of the second RAT, checking whether there-registration is successful, and if not ending the process.
 11. Themethod of claim 1, further comprising, after the registering with thenetwork of the second RAT, checking whether the re-registration issuccessful, and if not proceeding back to registering with the networkof the second RAT.
 12. The method of claim 1, wherein the first RAT isan evolved-universal terrestrial radio access (E-UTRAN) RAT.
 13. Themethod of claim 1, wherein the second RAT is a global system for mobilecommunications (GSM) RAT or universal terrestrial radio access network(UTRAN) RAT.
 14. The method of claim 1, wherein the circuit switchedcall is a voice call.
 15. A user equipment comprising a processorconfigured to: detect that a circuit switched (CS) call has beenrequested at the user equipment; if a most recent registration attemptor registration update attempt with a network of a first radio accesstechnology (RAT) resulted in an abnormal state, register with a networkof a second RAT; and initiate the CS call with the network of the secondRAT.
 16. The user equipment of claim 15, wherein the first RAT is apacket switched only RAT.
 17. The user equipment of claim 15, whereinthe user equipment is further configured to reselect to the network ofthe second RAT for the CS call.
 18. The user equipment of claim 17,wherein the reselecting to the network of the second RAT includesregistering with the second RAT.
 19. The user equipment of claim 17,wherein the processor is configured to register with the network of thesecond RAT even if a tracking area code or a location area code of alast successful registration matches the tracking area code or locationarea code in a broadcast message from the network of the second RAT. 20.The user equipment of claim 17, wherein the registration with thenetwork of the second RAT is in the form of a location area update thatutilizes a follow_on_request flag.